Auto racing's inherent dangers came front and center Oct. 6, when an accident launched Dario Franchitti's No. 10 machine into a fence during a race in Houston. He suffered spinal fractures leading to an announcement eight days later that he retired from racing. He won three Indianapolis 500s and four IndyCar Series championships. This story is from Nov. 14.

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With one simple news release, one of the greatest driving careers in IndyCar Series history came to an end Thursday.

"A sad day but I'm eternally grateful," Dario Franchitti said in a tweet after Ganassi Racing issued a statement announcing his retirement.

It felt hollow for Franchitti's news — that doctors advised him to retire after serious injuries Oct. 6 in Houston — to be delivered this way. But it shouldn't come as a surprise.

Franchitti suffered a concussion and spinal fractures that did not require surgery in the last-lap crash with Takuma Sato, and recovery was going to be slow.

Details of the medical advice haven't been shared, but Franchitti said the head injury was as much the issue as the spinal fractures. How many concussions he's had isn't known, but as the sports world is learning, they can have a cumulative effect.

"(Doctors) have made it very clear that the risks involved in further racing are too great and could be detrimental to my long-term well-being," Franchitti said in the release. "Based on this medical advice, I have no choice but to stop."

Franchitti could not be reached for comment. Indianapolis Motor Speedway historian Donald Davidson said he can't remember another 500 winner bowing out in this manner.

Said longtime Franchitti friend Tony Kanaan in a tweet: "As much as it hurts not seeing him compete again, I'm very happy that he got out of that accident and is still here with us."

Franchitti, who turned 40 in May, didn't need another head or back injury; he also didn't need another Indianapolis 500 victory or another IndyCar Series championship to validate his career.

He'll long be remembered as one of the best in both categories.

Only three drivers have won more Indy 500s, and only six have won as many as Franchitti. He had four series championships; only A.J. Foyt, with seven, has more.

In total race victories, Franchitti has 31, tied for eighth all-time.

The Scotsman retires as a world ambassador, someone as accomplished as his hero, Jim Clark, also a Scotsman, who won Indy in 1965 and died in a crash in Germany in 1968. It's said that the 500 makes a driver, but in this case the driver also has helped take the race to new destinations.

Franchitti won his first 500 in 2007 with Andretti Green Racing, his second and third with Ganassi Racing in 2010 and 2012. Driving for Ganassi, he stood a good chance of winning a fourth, something only Foyt, Al Unser and Rick Mears have accomplished.

"Simply put, Dario is a motor sports legend and will be sorely missed on the racetrack by everyone in the paddock and in the stands," team owner Chip Ganassi said in a statement. "His contributions to the sport of motor racing are too many to list, but I can tell you that they go way beyond what he has done on the track."

Ganassi noted that Franchitti has long been one of the sport's best students of history, appreciating "not only the actual science of the sport but also the rich heritage of those racers who have gone before him."

"You have so much to be proud of and I am proud to call u my friend!" former teammate Michael Andretti said in a tweet.

Ganassi said he expects Franchitti to continue with the sport in some form with the team. The next question is what happens to the Target-sponsored No. 10 car that Franchitti has driven since 2009.

Ganassi hired Kanaan last month to drive the No. 8 car sponsored by NTT Data, but Kanaan is a strong bet to switch seats. Ironically, Franchitti got the ride when Kanaan spurned Ganassi to remain with Andretti's team.